Method :1.Make about 300 ml of coffee syrup using the cold brew method .See previous posts or Google it .Briefly , 3 –4 tablespoons of coarsely ground coffee beans of decent quality (I like Columbian) in a plunger with about 350 ml water (cup and a half) . Put in fridge for about 12 hours . Push plunger home and decant clear coffee syrup .

2. Add sweetener . The coffee syrup is more bitter than ordinary coffee .Add enough for about 4 cups of coffee . To taste .Those fortunate ones that do not have to worry about sugar or carbs can use sugar .

3. Pour about 280 ml of the syrup into your 750 ml bottle . Top up with your alcohol of choice . Whisky , cane ,vodka etc .These are usually about 40% alcohol by volume. This is stated on the bottle .

4. Pour into a liqueur glass and enjoy !

Caffeine concentration :See Appendix A below .

Cold brewing gives a slightly higher caffeine concentration than hot brewing .The three tablespoons of ground coffee used above would be the rough caffeine equivalent of two cups of hot brewed coffee in the syrup . A bottle would then contain after dilution 281/468 *2 = 1.2 cups of hot brewed coffee caffeine .A liqueur glass is 25 ml . This then has the equivalent caffeine of 1.2*25/750 = 0.04 of a cup of hot brewed coffee (4 %) . This will not exactly keep you awake .

The really interesting thing here is whether this ratio of caffeine to alcohol balances the depressant effects of alcohol and the stimulant effect of caffeine at a stable level . Cf Irish Coffees .

Note the popularity of caffeine boosted mixers like Coke or Red Bull .

As an exercise , the Dear Reader can calculate the exact peak point of the interaction . A lot of experimental evidence will be required .Hint : Try shooters of coffee and tequila in varying ratio's .

The Right-Hand of the equation A(3) means the alcohol concentration of your liqueur. This is usually about 25% . Drinkable in small sips . Hence the liqueur glass .V(1) and A(1) means the Volume and Alcohol concentration of your booze . A(1) is typically 40% per volume . Cane , whisky , vodka , etc . See the bottle .

V(2) and A(2) means the Volume and Alcohol concentration of your Coffee Syrup . There is no alcohol , so A(2) =0 .

The left-hand of the equation is the weighted average of the two concentrations , weighted by volumes .